MOD Upgrading the ignition backup battery in a Carbon Cub E/FX-3 (EarthX ETX104)

[Neal]

I have two spare fuses once I learned of that as well and keep it in my spares kit.

Okay, I returned to the plane today for more ground run testing.

1) Pulling the emerg ignition breaker drops the voltage to 0 in the GDU

2) Pulling the inline fuse at the battery positive does nothing to the GDU

I need my clamp meter to measure amps for the charge, I looked at buying a mini meter with a display or a bluetooth option which would require a shunt install like in my RV but decided not to.

AMPS today during the ground run did not exceed 11. I will never be in a condition where the starter and/or ignition backup are not in a nearly full state of charge. With pigtails on both, they will be maintained and charged after any period of non use.

I've decided to call this MOD done. I'm staying with the EarthX ETX104. I will fly with the right forward panel off a few flights to monitor for my own comfort, both the electrical but also my mount solution.

 

[Cactus Charlie]

1) Pulling the emerg ignition breaker drops the voltage to 0 in the GDU

That is unexpected and seems to be in conflict with the published schematic.

IGN VOLTS is sensed at ignition switch terminal 5 by wire A11A20O which feeds the GEA 24.

With ignition switch Off terminal 5 is connected to the battery via switch jumper 5-3. Terminal 5 should see battery voltage when the ignition CB is pulled.

2) Pulling the inline fuse at the battery positive does nothing to the GDU

As expected since, with switch Off, switch terminal 5 is connected to switch terminal 4 and reads MAIN bus volts when the fuse is missing.

 

[ve6yeq]

As part of other design work, I calculated the book endurance of different backup batteries. The load I considered is a single Lightspeed Plasma III configured for an IO-360, with a rated power consumption of 1.3 A at 13.8 V. Since power consumption should be approximately constant with voltage, I assume it is 13.8 V * 1.3 A = 18W.

For Lithium batteries, the discharge voltage curve is very flat at about 13.0 V so 18 W / 13.0 V = 1.4 A.

So looking at the EarthX ETX 104, we have 4 Ah rated capacity: 4 Ah / 1.4 A * 60 min/h = 171 min. Batteries are normally considered EOL when the actual capacity is 80% of the rated capacity: 171 min * 0.8 = 136 min. Looking at the EarthX Technical docs, they suggest that capacity is reduced by 10% at -10 degC (winter flying inside the side panel): 136 min * 0.9 = 122 min. The book suggests we should see a practical endurance of 2 hours, which easily meets the 30 min IFR requirement.

If I repeat this for the TCW IBBS 3 Ah powering only the right ignition, I get an endurance of 91 min. Once again, 1.5 hours is clearly enough.

However, if I repeat this for the PowerSonic PS-1221 AGM battery, the calculations become slightly more complex because of the faster voltage drop during discharge. The 18 W constant-power load is 3 W per cell (6 cells in a 12 V AGM battery). According to the PS-1221 documentation, the constant power table indicates a 45 min endurance for a fully charged battery. Using 80% rated capacity as end of life reduces this to 36 min, and a -20% for -10 deg C (AGM batteries are much worse in cold temperatures than Lithium) results in just 28 min. Just less than the 30 min IFR requirement.

Given that I have always assumed book values are best-case numbers, and that, in my experience, with flying only a couple of hours a week, the PowerSonic battery is never fully charged, given its self-discharge rate and the time required to charge to 100% (28+ hours). I will be upgrading the AGM battery before I go too far from a precautionary landing site.

 

[Neal]

That is unexpected and seems to be in conflict with the published schematic.

IGN VOLTS is sensed at ignition switch terminal 5 by wire A11A20O which feeds the GEA 24.

With ignition switch Off terminal 5 is connected to the battery via switch jumper 5-3. Terminal 5 should see battery voltage when the ignition CB is pulled.

Give it a try on your plane and see if the results are the same with the breaker (no sense in testing the fuse)

Given that I have always assumed book values are best-case numbers, and that, in my experience, with flying only a couple of hours a week, the PowerSonic battery is never fully charged, given its self-discharge rate and the time required to charge to 100% (28+ hours). I will be upgrading the AGM battery before I go too far from a precautionary landing site.

Excellent write up @ve6yeq - great information.

I think anyone that chooses NOT to upgrade the PowerSonic battery should at least consider wiring in a charge setup so that a NOCO or similar AGM battery tender can be connected to ensure the battery is maintained and fully charged when ready to fly. They should already be ensuring the starter battery is charged for any periods of non use, this is your first level alternator failure protection.

Great discussion on this forum and a lot learned by me and others stemming from the N40DT discussion. I personally have improved my G3X alerting and understanding of the systems and what to do in the event of an ALT failure to force loads to the IBBS, etc. Thanks all that participated and helped to educate those watching.

 

[Cactus Charlie]

The load I considered is a single Lightspeed Plasma III configured for an IO-360, with a rated power consumption of 1.3 A at 13.8 V. Since power consumption should be approximately constant with voltage, I assume it is 13.8 V * 1.3 A = 18W.

I would expect the current drawn by each ignition module to depend on engine rpm. The higher the rpm the greater the current.

Ignition battery current is instrumented in my FX-3 but that gives me ignition module current only when emergency ignition is active. The highest value I have recorded is 2.0 A but it is more typically around 1.2 A- 1.3 A at run-up rpm.